When an air-vapour mixture is heated or cooled, without the addition or removal of moisture, the resulting "process" may be plotted as a horizontal line on the chart. . We don't actually perceive RH itself, we perceive the dampness or dryness of our bodies in reaction to ambient RH, or we perceive the effect on objects such as paper or cloth, which become damp or dry in response to the RH. ). Shelving or cabinets placed against exterior walls suffer high RH during cold weather. Some have even cracked audibly in their first winter in a heated museum (wooden folk art) while the collection manager sat quietly at their desk! 1, which shows that air from outside at 20°F and 100 per cent relative humidity will, when heated to 75°F, have only 12 per cent relative humidity. If the material is free to expand and contract as RH goes up and down, then there is no problem, but if the material is constrained by other components of the object, or simply by its own inner bulk during a rapid fluctuation, then expanding parts will be crushed, and shrinking parts may fracture (as with the paint in Figure 3). Periods of high humidity in summer are controlled by small portable dehumidifiers. Uneven temperature across a package leads to damp, even condensation, at the end that is consistently colder than most of the package. Corroded metals lose their original surface. A summary of all forms of deterioration due to incorrect RH, and the sensitivity of various collections to each one. Roofs for such conditions present a special problem. Again, why bother with the elaborate mechanical control? Any object known to have been at least once at some very low RH, say 10% RH, or at least once at some high RH, say 80% RH, is not susceptible to further mechanical damage from one more event of the same magnitude, since any fractures, delaminations and irreversible compressions will have already taken place (unless it is known to have weakened significantly from other causes in the interim). Often, one will find that only a small and manageable part of the collections is at significant risk from incorrect RH. Most of the types of incorrect RH depend on the pattern over time. Although engineers that consult on climate control systems may use other humidity parameters such as vapour pressure or dewpoint temperature (Consult the glossary), all the deterioration issues of collections that are linked to humidity, and all the specifications one can provide for consultants about caring for collections in terms of humidity, are best expressed using RH as the parameter. The most important form of response, in museums both large and small, is human. Consideration should always be given to the difficulties that may be created in any given building before humidification is added. For more information consult Waller ( Thirty years ago, humidity control for Canadian collections seemed a combination of simple specifications and high, unsustainable costs. It rarely gets really warm here and you can safely leave your swimwear at home. 1994 Active machinery – humidifiers and dehumidifiers – respond minute by minute via their humidistats. The converse is that when cold air is heated, the RH falls. Daly Hartin, D. 2007 Current best estimate for stable film materials to remain unchanged. As with avoid, whenever block is used as a tactic, one must check the results from time to time by monitoring RH, i.e., "detect". The wood of the stretcher bars can moderate the RH in their immediate vicinity for about 30 hours, no more, so they can eliminate the daily fluctuations in the painting layers nearby, but not the seasonal. Very low RH in Canadian museums only occurs with indoor heating. High risk of mechanical damage to high vulnerability objects, moderate risk to most paintings, most photographs, some objects, some books and tiny risk to many objects and most books. Neglecting natural moisture sources as much as 9.6 gallons of water per day will have to be added to maintain 40 per cent relative humidity at 75°F for each 10,000 cubic feet of building volume, for one air change per hour with outdoor air saturated at 20°F. Although the physical phenomena that underlie damage from RH fluctuation are analogous to those discussed under fluctuations in the section on Incorrect temperature, the collections that are vulnerable are not at all the same. The average Canadian dwelling will have a transmission heat loss of about 0.04 Btu per cubic foot of building volume per degree temperature difference between indoors and outdoors. IIC, ( 2007 This can be achieved by considering Table 1, and the section on deterioration. This video was created by the Canadian Conservation Institute. most oil paintings on canvas. It is important, therefore, to be accurate about assessing past climate control, and not to underestimate how bad it was, since the worse one knows the past to have been, the easier it will be to make the future better (the same is shown for temperature fluctuations). Unstable glass (such as the trade beads in Figure 2) "sweat" when the RH is above a critical RH (~55% RH) because fluxing compounds in the glass deliquesce, on the other hand they "crizzle" when the RH is below the critical RH (~40% RH) that causes dehydration of other compounds in the glass. Cabinets placed against south facing windows may protect the objects from sunlight, but the result is a heated, very low RH interior. The afternoon humidity is for 3:00 pm local standard time. GLOSSARY Absolute humidity . (ASHRAE, On the other hand, by the time we reach the respond stage, which includes building and running all kinds of active and passive climate control systems, the designers of such systems need a "correct" RH. This page includes a chart with historical data for Canada Average Temperature. All the numbers here are based on weather data collected from 1981 to 2010. The water vapour content of the air at saturation at various temperatures is shown by the saturation line of Fig. When the temperature is changed, the relative humidity changes, since the capacity of the air for holding moisture increases with increasing temperature. Commercial signs painted on metal. It will be well protected from all RH fluctuations, i.e., it will "buffer itself." Figure 4 explains most of these differences. This creates a high risk of the cracking of furniture (as in Table 1) but there is an important qualification: only if the low RH is maintained long enough for the furniture to fully respond. Environment Canada. Table 1. Originally published January 1960. 25-50. Environment Canada. Staff must learn to recognize the various incorrect RH, and begin to assess the risks to the collections, using Table 1. A measure of humidity in terms of the pressure exerted by the water vapour (which contributes to the overall air pressure). some 18, A small fraction of minerals, such as hydrates and pyrites have specific critical. The tables give the month's daily average relative humidity.
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